Lunar Prospector Neutron Spectrometer (NS)

Description

The Lunar Prospector Neutron Spectrometer (NS) is designed to detect small amounts of hydrogen which may exist on or near the Moon's surface. Hydrogen is assumed to primarily indicate the presence of water ice, in which case the NS can detect water ice at a level of less than 0.01%. The Moon has a number of permanently shadowed craters near the poles with continuous temperatures of -190 degrees C. These craters may act as cold-traps of water from incoming comets and meteoroids. Any water from these bodies which found its way into these craters could become permanently frozen. The NS will also be used to measure the abundance of solar wind implanted hydrogen. Additionally, the NS can be used to map Gd+Sm abundance through detection of anomalous reductions in leakage fluxes of thermal neutrons. Achievement of these objectives requires separate measurements of thermal, epithermal, and fast neutrons.

The Neutron Spectrometer is a thin cylinder colocated with the Alpha Particle Spectrometer at the end of one of the three 2.5 meter radial Lunar Prospector science booms. The instrument has a surface resolution of 150 km. For the polar ice studies, the NS will examine the poles to 80 degrees latitude with a sensitivity of at least 10 ppm of hydrogen. For the implanted hydrogen studies, the NS will examine the entire globe with a sensitivity of 50 ppm. The NS consists of two identical Reuter Stokes helium 3 gas proportional counters. Each counter is a cylindrical cannister 5.7 cm in diameter and 29 cm long, 20 cm of which is the active length, filled with 10 atmospheres of helium 3. Any neutrons colliding with the helium atoms will give an energy signature which will be detected and counted. One of the cannisters is wrapped in a 0.63 mm thick sheet of cadmium and one in a 0.63 mm thick sheet of tin. The cadmium screens out thermal (low energy or slow-moving) neutrons of energies less than about 0.4 eV while the tin does not. Thermal neutrons are cosmic-ray generated neutrons which have lost much of their energy in collisions with hydrogen atoms. Differences in the counts between the two cannisters indicate the number of thermal neutrons detected, which in turn indicates the amount of hydrogen on the Moon's crust at a given location. Large quantities of hydrogen could be due to the presence of water. The two counters share a cylindrical axis which is perpendicular to the boom and the spacecraft spin axis. The two counters are biased by separate 2 kV power supplies having 256 levels of voltage control. A pair of charge-sensitive preamplifiers and 8-bit analog-to-digital converters are used to characterize the pulse height for each detected event to determine if it falls within the energy interval bracketing the 765 keV peak for the helium-3/neutron reaction. These are recorded on a 16-bit counter and read out every 0.5 seconds.